Guide bushings play an important role in precision drilling and reaming operations. Typically, guide bushings are annularly shaped elongate elements formed of steel. They are usually fitted into a drill plate that is designed to be securely held adjacent to the workpiece that is to be drilled or reamed. The bore of the bushing acts to ensure straight entry and exit of the rotating member, typically a twist drill or reamer. (While the rotating member is hereinafter referred to as a twist drill or drill, it is to be understood that a bushing made in accordance with this invention could also be utilized with a reamer.)
Shavings of the workpiece are formed by the shearing action of a rotating drill as it advances through a workpiece. These shavings are most commonly referred to as chips. The chips are sometimes continuous, but often in the form of several pieces, depending upon the material being drilled or the design of the drill. During a drilling operation using a guide bushing, problems are encountered when chips rising through the flutes of the drill become compacted in the guide bushing. Compaction of chips in the guide bushing leads to compaction of chips in the workpiece which results in an undesirably rough surface finish of the hole being drilled. The compacting of chips in the bushing has a binding effect on the drill. The binding effect of the chips increases the amount of torque that must be applied to the drill and can lead to drill stalling, excessive drill and bushing wear, or drill breakage. Wearing of the guide bushing results in difficulties positioning the drill in order to form the precise hole required.
Methods currently used in attempts to solve the chip compaction problem generally include backing the drill out of the guide bushing to allow the chips to fall away from the drill and the bore of the bushing, or modifying the drill by altering the dimensions of the drill lands and flutes. Another alternative is to configure the drill plate that holds the bushing so that the bushing is spaced away from the workpiece. The space between the bushing and the workpiece allows most chips to separate and fall off the drill before entering the bushing. The disadvantage of such a drill plate configuration is that a space between the workpiece and the bushing is detrimental to precision drilling. Precise drilling operations require that the bushing be placed in direct contact with the surface of the workpiece for best control of the drill as it enters the workpiece.
In addition to the foregoing attempts to solve the chip compaction problem in guide bushings, attempts have been made to design bushings that avoid the problem. U.S. Pat. No. 1,612,215 issued to Muth describes a bushing that is intended to avoid compaction problems by breaking up chips so that they may either be directed upward or fall down out of the way of the bushing. Again, in order to fall out of the way of the bushing, there must be a space between the bushing and the workpiece, resulting in the previously discussed diminished drilling precision. Muth uses a bushing having a groove extending nearly vertically from one end of the bushing to the other. As the drill is rotated within the bushing, a scissor-like action is created between the leading edges of the drill lands and the bushing groove. This action causes the chips to be chopped or broken up into smaller pieces. The major problem with the Muth bushing is that the chips that are chopped or broken by the groove tend to compact and bind in the flutes of the drill located within the wall of the bushing. This binding impedes the upward movement of the chips through the bushing causing further compacting and binding. Chips are especially prone to bind in the Muth bushing if the bushing is held--as it should for precise drilling--in contact with the workpiece. In this case, all of the broken chips must be forced through the bushing since they are unable to fall away before entering the bushing. Thus, Muth does not completely solve the chip compaction problem of earilier bushings. Furthermore, the above-described scissor-like action between the groove and the drill will cause rapid wearing of the drill.
For precision drilling it is also to use as long a bushing as possible since a longer bushing provides greater control of a drill positioned within the bushing's bore. The previously mentioned compaction problem limits the length of bushing that can be utilized since the longer the bushing the greater the chance that compaction will occur as the chips travel up the length of the bushing. Most bushings commonly used today utilize a bore length that is approximately two to three times the internal diameter of the bore. This length is selected so that the bushing can adequately guide the drill in a straight line, without being so long as to aggravate the chip compaction problem.
This invention provides a bushing that effectively removes the chips from the workpiece while avoiding the compaction problems just described. Due to the effectiveness of the chip removal capability of a bushing made in accordance with this invention, the bushing may be placed in direct contact with the workpiece and be significantly longer than currently used bushings. Additionally, the bushing of this invention obviates the need for the time consuming process of backing the drill out of the bushing during the drilling operation.